The present invention relates to a method and apparatus for removing silver and copper ions from solutions, e.g. a silver replacement process purge stream. In another aspect the invention relates to a method of separating silver and copper by selectively electrowinning silver at a higher cathode/reference reduction potential than that of the copper. In yet another aspect, the method provides for electrowinning copper ions from a silver-depleted stream that is substantially diluted.
In the electrolytic refining of silver as well as in silver coating processes wherein silver is deposited on a copper electrode or electrolessly deposited from cyanide solutions or ammoniacal silver solutions, the rate of silver deposition is retarded in proportion to the concentration of copper in the solution. Traditionally, low copper concentrations in silver replacement solutions or in silver refining process solutions has been an important goal due to the interference of the copper ions with the silver covering or plating quality.
More recently, contrary to the prior art admonitions against high levels of copper in a replacement solution, it has been found that resulting deposited or plated silver can present a surprisingly brighter appearance with substantially lower surface resistivity and improved oxidation resistance if the replacement solutions include measured amounts of copper. In either case, whether the copper is an unwanted contaminant or a controlled amount of copper is desired, silver replacement process purge streams or purge streams from silverplating refining processes contain substantial amounts of copper ion along with silver ions. Such concentrations present substantial problems in removal of the copper and silver ions, for example, copper ion forms in the presence of high concentrations of silver ions which leads to the precipitation of silver metal and the formation of copper. Precipitation of silver causes sludge formation, poor electrode life time and high silver discharge from ion exchange procedures.
Control of copper ion concentrations within silver refining procedures or silverplating procedures requires improved methodologies for separating both silver and the copper ions from each other and from process solutions. For example, in working with a silverplating bath waste solution containing both copper and silver ions, effluent from such processing requires significant reduction of such ion contents before discharge limits can be reached.